The present invention generally relates to scanning lenses as well as scanning apparatus using a scanning lens, and more particularly to a scanning lens as well as a scanning apparatus using the scanning lens which may be applied to laser printers, digital copiers, laser facsimiles and the like.
A conventional scanning apparatus in which a beam of light is deflected by a deflector unit and the deflected light is scanned over, for example, a surface of a photosensitive body, has been widely applied to the laser beam printers, the digital copiers, or the laser facsimiles. In such a scanning apparatus, a scanning lens, or an f.theta. lens, is used for focusing the light being deflected by the deflector unit onto the surface of the photosensitive body as scanning spots or picture elements to form an electrostatic image on the surface of the photosensitive body. There are several proposed types of conventional f.theta. lenses. One of those conventional f.theta. lenses is designed to carry out an approximate f.theta. function and is constructed as a single lens, as disclosed, for example, in Japanese Patent Application No.62-304737 which is one of the priority documents of commonly owned U.S. Pat. No. 4,934,772. This conventional f.theta. lens performs a wide range scanning, but there is a tradeoff between a scanning linearity and a correction of curvature of field. That is, as the scanning linearity becomes better, the curvature of field becomes worse to a level that cannot be corrected by electric signal processing sufficiently, and it is difficult for the conventional f.theta. lens to provide a high density of the scanning spots.